OFDM is becoming increasingly popular in design of high data rate communication systems. In OFDM systems, a single high rate data stream is divided into several parallel low rate sub streams, with each sub stream being used to modulate a respective sub carrier frequency. This is generally accomplished by broadcasting the high rate data using a transmitter to a receiver from one or more essentially identical transmitters via multiple air interface paths.
The transmitter in an OFDM system receives a stream of digital data bits to be transmitted. The received digital data bits are then scrambled and convolutionally encoded per well-known principles. The data bits are then sent to an interleaver to interleave the bits. The interleaved bits are then signal-space mapped to produce a stream of complex symbols. After mapping, the stream of complex symbols is then transformed to time domain by an inverse fast Fourier transformer (IFFT). Guard intervals are then established and the data bits are transmitted.
The receiver then receives the transmitted data bits and sends them to a guard interval remover, which removes the guard intervals inserted by the transmitter. From the guard interval remover the data bits are then sent to a fast Fourier transformer (FFT) for transforming the signal back into frequency domain. The FFT then outputs a stream of complex symbols in a single sequence of data bits. The complex symbols are then demapped and inputted into a de-interleaver for reordering bits into the order they were in before being interleaved by the interleaver of the transmitter. De-interleaving is a reverse process, which restores the original sequence from the interleaved sequence. Next, the deinterleaved data bits are sent to a decoder for decoding the bits in accordance with the convolutional coding schemes well known in the art.
The interleaver at the transmitter, in such OFDM based communication systems, scrambles a signal over a certain time interval, by reordering the data bits. Typically, block interleavers are employed, where a signal is scrambled by writing the data bits into rows and reading them out in columns, in a known manner. If block-coded symbols are interleaved over a time duration of many blocks before transmission, symbols associated with a lost packet will be deinterleaved by the receiver and found among many different coded blocks.
Also, in OFDM-based communication systems, the de-interleaver at the receiver has to be synchronized to the interleaver. In order to synchronize the de-interleaver to perform the de-interleaving, the data bits coming from the demapper have to be first stored in a buffer and then the de-interleaving has to be performed. The time required to fill the buffer with the data bits in order to synchronize the de-interleaver can be significant and can affect the synchronization. This can result in not meeting the receiver processing time generally required by a packet based communication system. This can degrade the performance significantly. In addition, the amount of memory required to store the data bits in the buffer can be significant. Further, such process of storing the data bits in the buffer and performing the de-interleaving operation by retrieving the stored data bits can result in a significant delay, overhead information, additional processing, and synchronization problems.